(1) Field of the Invention
The present invention relates generally to a pre-stressing sheath for preventing wet concrete from penetrating between the sheath and strands being debonded. In particular, the present invention relates to plastic clip-on type sleeves used to encircle and debond concrete reinforcing members such as cables.
(2) Description of the Prior Art
Pre-cast concrete construction elements such as bridge girders, etc., are stressed by placing tension on an elongated cylindrical rod or cable bundle having a generally circular cross-sectional area normally formed of twisted wires generally referred to as strands. Strands that are placed in a straight configuration near the bottom of a pre-stressed girder tend to overstress near the end of the girder at the moment of de-tensioning. In order to avoid this overstress, several strands are usually draped upwardly near each girder end to reduce the eccentricity of the force they cause about the centroid of the girder. The effort required to position and stress the draped strands is expensive, time consuming, difficult and dangerous.
Recently, a less expensive, faster, easier and safer technique for eliminating overstress has been developed and used commercially to prevent the undesirable overstress. The improved procedure, places and stresses the strands necessary for flexure at mid-span of a girder in a straight configuration extending over the entire length of the girder. The overstress near the ends of the girder at the moment of de-tensioning is avoided by debonding several strands from the end of the girder towards mid-span over a length which would otherwise be overstressed. This eliminates the eccentric forces that the debonded strands would otherwise generate about the centroid of the girder, and thus reduces stresses.
Generally, the debonding of one or more strands is accomplished by placing a debonding sheath about the strands to be debonded. Once the sheath is installed on the strands, the ends of the sheath are sealed to help prevent the strands from coming into contact with a mortar that is used to bond unprotected strands to the girder's concrete mass. At present, the preferred debonding sheath is a plastic tube having a longitudinal slit that allows the tube to be clipped over the strands to be debonded. However, because of the longitudinal slit, this type of sheath is not as protective in preventing wet mortar from coming into contact with the strands to be debonded. Therefore, while the split debonding sheath is currently favored, it by itself does not offer an adequate strand debonding solution.
Another attempt to prevent mortar from coming into contact with strands uses a hollow cylinder having a continuous wall that slides over the strands to be debonded. While a hollow cylinder having a continuous wall offers the best protection for preventing wet mortar from coming into contact with the strands, it is difficult and sometimes impossible to apply. What is needed is a pre-stressing sheath that offers the debonding protection of a hollow cylinder having a continuous wall along with the ease of use of a split debonding sheath.